X-ray CT systems irradiate the subject with X-rays and acquire image data (hereinafter referred to merely as “data”) that represent a view in a predetermined sectional plane of the subject, on the basis of the X-rays that have passed through the subject.
An example of a method of X-ray irradiation is involved in shuttle scanning, where the X-ray CT system performs scanning while the top plate with the subject is being reciprocated between the starting point and the ending point of a set scanning range (e.g., patent reference 1).
There is a case where a scanning duration is set for the X-ray CT system, where the scanning duration is a time that the top plate takes to make a reciprocating movement in connection with X-ray irradiation. The scanning duration includes an acceleration time, a constant-speed time, and a deceleration time, along with a waiting time (interval), during which the top plate is stationary. Incidentally, the acceleration time is the time when the top plate is being translated by the X-ray CT system in accelerating mode; the constant-speed time is the time when the top plate is being translated at a constant speed by the X-ray CT system; and the deceleration time is the time when the top plate is being translated by the X-ray CT system in decelerating mode. In addition, during the waiting time, the X-ray CT system performs such actions as offset correction on the data, directional switching of the top plate, input and output of control signals, tracking agreement (described later), measurement of the load that is caused by the subject.
If an ADCT (Area Detector CT) is applied to the X-ray CT system, then dynamic scanning is possible in which a certain scanning range is being scanned at once. However, even without the application of an ADCT, pseudo-dynamic scanning can be accomplished by the use of shuttle scanning. Shuttle scanning in this case is used mainly for observation of chronological changes in the subject.
In this type of shuttle scanning, data are acquired during the time s of the movement of the top plate from the starting point to the ending point of the scanning range, and data are also acquired during the time t of the movement from the ending point to the starting point. Furthermore, in the shuttle scanning, differential images are generated from the data sets that have been acquired respectively during the corresponding different times of the movement, for observation of chronological changes. For observing chronological changes by using differential images, therefore, it is important that the differences between the respective data sets be accurately captured for the corresponding different movements, for example, during the movement times s, t.
In such dynamic scanning as done with the ADCT, data are acquired in a lump for a predetermined range of the subject, and the circular orbit taken for the scanning is uniform even at different time points. By contrast, in the shuttle scanning, generally, the scanning tracks are different for the data set acquired during movement time s and for the data set acquired during movement time t. For example, when the scanning track is clockwise in the direction of the movement from the starting point to the ending point, the scanning track turns counterclockwise, i.e., in the direction of the movement from the ending point to the starting point. Because of this reason, the differential image that is created from the data sets achieved during movement time s and movement time t may include differences that result from factors other than chronological changes. Such differences, if any, adversely affect the algorithmic analysis that follows in the process.
For example, if differential processing is executed for two data sets that have been acquired for different scanning tracks, then the resultant differential image may include an artifact for one data set that has occurred during one scanning time and another artifact for the other data set that has occurred during the other scanning time. If the difference shown in the differential image is extremely small or is a little different in luminance from the part other than the differential, it is difficult for the examiner to distinguish between real differences and artifacts that have appeared in the differential image.